The present invention relates to a method for coating a surface of a titanium implant, which has been widely used as a grafting material, with low crystalline hydroxyapatite having bioabsorbability. Further described is an implant coated by the same method.
In recent years, hydroxyapatite has been widely used as an artificial biomaterial capable of replacing hard tissues such as bones or teeth. Hydroxyapatite is a material that is chemically and crystallographically identical to mineral components which constitute bones and teeth in the human body. When transplanted into the human body, hydroxyapatite exhibits high biocompatibility with the surrounding cells and rapidly forms a chemical bond directly with bones at the junction region. A pure hydroxyapatite crystal composed of calcium ions, phosphate ions, and hydroxyl ions is a stoichiometric crystal having a rod-like structure and has a high crystallinity. On the other hand, a biocrystal isolated from bones or calcified cartilage is a nonstoichiometric hydroxyapatite which has a low crystallinity [see
Titanium has been widely used as an implant material due to having physical properties similar to those of human bones and excellent mechanical strength. Further, titanium has been extensively used as a biological metal since it does not cause any inflammatory responses or other immune response in vivo. In order to impart bioactivity to titanium for industrial applications, the titanium is subjected to a variety of surface modifications such as blasting, acid etching, and the like (see numerous patent documents including Korean Patent Application No.
For these reasons, there have recently been developed a variety of methods for obtaining a biomaterial having both excellent mechanical strength and biocompatibility for use of replacement of biological hard tissues, by coating titanium with a hydroxyapatite thin film. Conventional representative methods of coating a ceramic, thereby imparting bioactivity to a surface of titanium, may include plasma spraying, sputtering, ion implantation, ion beam deposition, and the like. In order to form a crystalline film in which crystallographical properties of hydroxyapatite are similar to those of biocrystals, various approaches using a calcium phosphate solution or a simulated body fluid have been undertaken.
Plasma spraying which has been most commonly used among the above-mentioned methods has shortcomings such as non-uniformity of a coating layer due to instantaneous exposure to a high temperature of 10,000 °C or higher, and a difficulty to achieve a coating having a thickness of less than about 10 µm. Further, this method has a problem of very low biological reactivity in that hydroxyapatite coated on the surface of titanium undergoes decomposition in vivo due to a very high crystallinity or is refractory to removal by osteoclasts. Further, this method is known to involve simultaneous formation of calcium phosphates or calcium hydroxides having different phases as by-products [see
Further, sputtering or ion implantation, apart from high-priced equipment for this purpose, has suffered from various problems such as complex shapes, poor uniformity of the coating layer formed on irregular parts, and detachment of implants due to the peeling-off phenomenon in the human body.
On the other hand, as a wet coating method, there is a method using a calcium phosphate solution or a simulated body fluid. Preparation or coating of various types of calcium phosphates is started from a calcium phosphate ion solution. These calcium phosphate compounds can be prepared by mixing calcium ions and phosphate ions in an aqueous solution under a variety of conditions. In this connection, it is known that the type and form of compounds are greatly affected by ion concentrations, Ca/P ratios and pH conditions [see
Calcium phosphate compounds using a wet method have different equilibrium phases, depending on temperatures and pH values. In particular, at a temperature of 40 °C or lower, calcium phosphate has an equilibrium phase of amorphous (Ca3(PO4) · nH2O; n=3 to 4.5) or nonstoichiometric hydroxyapatite (Ca10-x(HPO4)x(PO4)6-x · nH2O; x=0 to 1, n=0 to 2) at a pH of 7 or higher, an equilibrium phase of octacalcium phosphate (OCP, Ca8H2(PO4)3 · 5H2O) at a pH of 6 to 7, and an equilibrium phase of dicalcium phosphate (DCP, CaHPO4), dicalcium phosphate dihydrous (DCPD, CaHPO4 · 2H2O) or the like at a pH of 6 or lower. The calcium phosphate coating according to a conventional wet method is based on change of calcium phosphate solubility, taking advantage of the fact that the solubility of calcium phosphate decreases as the reaction temperature increases. Therefore, an initial process should proceed at a low temperature of 2 °C to 5°C, and it is very difficult to obtain a coating film of calcium phosphate as well as a colloidal solution of calcium phosphate without the elevation of temperature. Further, during the process in which the colloidal solution and coating film of calcium phosphate are obtained by elevating reaction temperature, a pH of the solution is generally terminated in the range of 6.0 to 6.5. Even though an accurate equilibrium phase of the calcium phosphate coating film according to the conventional wet method has not been fully understood, it seems to be OCP based on the correlation between the temperature and the pH of the calcium phosphate solution.
As discussed above, the conventional wet method utilizes a difference in the solubility of calcium phosphate in response to changes in temperature, and has limitations requiring control of temperature and/or pH, elevating reaction temperature, and complex procedure for the formation of a calcium phosphate coating film.
The chief aim of the present invention is to overcome the problems as described above. Described herein is a low crystalline hydroxyapatite-coated implant which may be used for a long period of time while not being isolated from new bones such that the implant is absorbed by osteoclasts in vivo, thereby taking part in a remodeling process of bones.
Further, a coating method of low crystalline hydroxyapatite according to the present invention provides is relatively simple as compared to a conventional wet method. The coating method is also economically favorable due to no need for strict control of reaction conditions such as temperature and/or pH, and elevation of reaction temperature while imparting excellent bioactivity.
The present invention provides a method for coating an implant, wherein the method comprises: pretreating a surface of a titanium or titanium alloy implant, mixing a phosphate ion solution with a calcium ion solution to prepare a calcium phosphate solution, wherein the calcium phosphate solution has a concentration of 1.0 mM to 10 mM, dipping and storing the titanium or titanium alloy implant in the calcium phosphate solution for 1 hour to 3 hours at the temperature of 15°C to 30°C, wherein the resulting thin film of low crystalline hydroxyapatite has a network- or island-like morphology and is bioabsorptive.
The implant coating method according to the present invention has an advantage of being usable for a long period of time while not being isolated from new bones, in such a way that the implant is absorbed by osteoclasts in vivo, thereby taking part in a remodeling process of bones.
Further, the implant coating method according to the present invention is capable of enhancing a success rate of the implant in such a way that the bioactivity imparted to a surface of the titanium implant results in strong interfacial bonding between implant bone tissues and teeth tissues, and the promotion of osteoconductivity or osteogenesis. Furthermore, the separation of the implant arising from peeling of the surface coating layer is prevented due to bioabsorbability of low crystalline hydroxyapatite forming a coating layer.
The implant coating method according to the present invention enables the production of an excellent dental implant by a very simple process at a low cost, and the control of thickness and morphology of the coating layer as necessary, such as a thin coating layer, a thick coating layer, and co-exposed surface of titanium and coating layer. Therefore, the present invention can be applied to a variety of biomaterial field in accordance to desired uses, for example, the orthopedic area as well as the dental area.
Objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
The present invention relates to a method in which hydroxyapatite having excellent bone growth and osteoconductivity is coated on a surface of titanium having excellent mechanical properties for the use of implant. For this purpose, the present invention provides a technique which induces a growth of network-like or island-like low crystalline hydroxyapatite on a surface of a titanium implant in a low-concentration calcium phosphate aqueous solution at room temperature of 15 °C to 30 °C.
The coating method according to the present invention is also applicable to implants using a titanium alloy other than pure titanium, as a raw material, or implants using titanium as a raw material and then being surface-treated by a variety of methods. Even though the process time may be slightly different from the embodiment of the present invention, depending on the type of surface treatments, the reaction time can be easily controlled by those skilled in the art to which a technique of coating an implant surface with hydroxyapatite pertains.
Specifically, the implant coating method of the present invention includes:
The pretreatment step may be carried out by a variety of methods commonly used in the art, such as a Resorbable Blasting Media (RBM) method in which surface treatment is carried out using resorbable blasting media, a Sand-blasted Large grit, and Acid etched (SLA) surface treatment method in which aluminum blasting is followed by acid etching, heat treatment at a temperature of 300 °C or higher, anodization, and heat treatment of surface after acid or base treatment. When a titanium or titanium alloy material which was surface-treated by any of the above-mentioned methods is used, the pretreatment step may be omitted. Further, the pretreatment step preferably includes ultrasonic treatment of the titanium or titanium alloy implant in distilled water, a calcium ion solution, a phosphate ion solution or a calcium phosphate solution for 1 minute or more. The ultrasonic treatment may also be carried out after the above-mentioned RBM, SLA or heat treatment and the like is performed.
In the pretreatment step, with regard to an implant composed of the above-mentioned titanium or titanium alloy as a raw material, any hydrocarbon remaining on a surface of the implant may be optionally removed using a nitric acid solution. The concentration of the nitric acid solution is preferably 4 v/v% to 60 v/v%. Depending on hydrophilic properties and morphology of the surface, the nitric acid cleaning process may be omitted. Following the nitric acid cleaning, the surface may be washed with distilled water in order to remove residual nitric acid on the surface. The distilled water washing may also be omitted for the surface which was not subjected to the nitric acid cleaning process.
A low-concentration (about 1.0mM to about 10mM) calcium ion solution proposed in the present specification can be prepared by a simple method of dissolving calcium nitrates(for example, Ca(NO3)2 or Ca(NO3)2 · 4H2O, etc.) or calcium chlorides (for example, CaCl2 or CaCl2 · 2H2O, etc.) in distilled water. A low concentration (about 1.0mM to about 10mM) phosphate ion solution of the present application can be prepared by a simple method of dissolving ammonium phosphates (for example, (NH4)2HPO4, etc.) or sodium phosphates (for example, Na2HPO4 or Na2HPO4 · 2H2O or Na2HPO4 · 7H2O, etc.) or potassium phosphates (for example, K2HPO4 or K2HPO4 · 3H2O, etc.) in distilled water.
Prior to application thereof to the implant substrate, the phosphate ion solution and the calcium ion solution are mixed to prepare a calcium phosphate solution which is then applied to the implant substrate.
The concentration of the calcium phosphate solution is in the range of 1.0mM to 10mM. The dipping time of the implant in the low-concentration calcium phosphate solution is 1 hour to 3 hours, the dipping time may vary depending on intended state and morphology of the surface, and a coating thickness. Further, the dipping temperature is in the range of 15°C to 30°C. 15°C to 30°C, The dipping temperature may be kept constant or elevated. Depending on intended uses and purposes, the dipping time and dipping temperature can be easily selected by those skilled in the art.
The calcium ion solution and the phosphate ion solution, which were prepared at a low concentration, are mixed to prepare a calcium phosphate solution which is added to the titanium implant, followed by dipping for 1 hour to 3 hours at the temperature of 15°C to 30°C, and the titanium implant is removed and briefly washed with distilled water, whereby low crystalline hydroxyapatite having network- or island-like morphology can be coated on a titanium or titanium alloy implant surface.
The coating method according to the present invention has advantages in that there is no need to control a pH of the calcium phosphate solution. It is not necessary to stir for mixing of the phosphate ion solution and the calcium ion solution.
The method of coating low crystalline hydroxyapatite proposed in the present invention is based on sufficient utilization of a phenomenon in which a solubility of the calcium phosphate solution is lowered at room temperature and a chemical mechanism that mixing calcium ions with the phosphate ion solution causes changes in a dissociation constant of hydrogen phosphate (HPO42-) in the phosphate ion solution and therefore amorphous or crystalline calcium phosphate can be continuously produced during dipping of a titanium implant in the calcium phosphate solution. The following is a reaction scheme of the phosphate ion solution.
Na2H(PO4)2 → 2Na+ + H(PO4)2-
H(PO4)2- ↔ H- + (PO4)3- (Ka = 10-11.66)
A dissociation constant of hydrogen phosphate ions in the phosphate ion solution is very low. Therefore, there is a very small amount of a dissociated form of hydrogen phosphate ions in the solution. Unlike the conventional method which takes advantage of a buffer or temperature-dependent solubility of calcium phosphate, the present invention makes use of changes in dissociation constant of hydrogen phosphate ions occurring during a mixing process of the calcium ion solution and the phosphate ion solution. An addition of the calcium ion solution to the phosphate ion solution accelerates the dissociation of hydrogen phosphate ions. Even though the dissociation of hydrogen phosphate ions takes place continuously and may vary to some extent depending on the concentration of the ion solutions or reaction temperature, the dissociation of hydrogen phosphate ions takes place rapidly around one hour after proceeding the reaction, and then the dissociation of hydrogen phosphate ions takes place slowly and continuously as time goes on. Accordingly, it is possible to obtain a coating film having a network-like morphology at the point where rapid dissociation of hydrogen phosphate ions takes place. When the process is allowed to proceed for a longer period of time (3 hours or more), it is possible to obtain a thick coating film of low crystalline hydroxyapatite due to continuous dissociation of hydrogen phosphate ions. The thin film of low crystalline hydroxyapatite has a thickness of about 10 nm or less. As dipping time increases, a thicker film can be obtained.
Pretreatment in the present invention is a process which imparts activity to the implant surface in order to secure uniform and high-adhesion coating of calcium phosphate crystals to a surface of the titanium implant in the calcium phosphate solution. Generally, a production rate of calcium phosphate crystals is very high in the calcium phosphate solution at room temperature. Therefore, if appropriate activity is not imparted to the titanium implant surface, a coating is not formed on the implant and/or adhesive strength of a coating to the implant is too lowered. In conclusion, the pretreatment of the titanium implant surface is needed to increase a generation and growth of crystalline calcium phosphates on the implant surface. Further, the pretreatment results in improved coating uniformity of low crystalline hydroxyapatite on the implant surface, and significantly improved repeatability and reproducibility.
The pretreatment method in the present application is carried out as described hereinbefore, preferably, by subjecting the titanium or titanium alloy implant to ultrasonic treatment in distilled water, a calcium ion solution, a phosphate ion solution or a calcium phosphate solution for one minute or more.
Low crystalline hydroxyapatite prepared by the coating method according to the present invention is characteristically nonstoichiometric and is also bioabsorptive.
Further, the low crystalline hydroxyapatite-coated implant described herein has a double surface with co-exposure of a network- or island-like coating layer and an implant titanium surface, whereby osteogenic capacity is improved due to synergistic effects between adhesion of titanium to cells and osteoconductive effects of hydroxyapatite.
As described herein, the coating of low crystalline hydroxyapatite having network- or island-like morphology in the calcium phosphate solution can determine a desired thickness depending on hydroxyapatites concentration, temperature and process time. A state of the coating can be confirmed under a scanning electron microscope. Due to the simplicity of the process, a coating degree of low crystalline hydroxyapatite can be appropriately controlled corresponding to desired applications in various implant areas as well as in teeth.
Now, the present invention will be described in more detail with reference to the following Examples.
An SLA (surface treatment method including aluminum blasting followed by acid etching)-treated implant was dipped in a 12% nitric acid (HNO3) solution for 5 minutes, followed by ultrasonic treatment, and the acid remaining on a surface of the implant was removed using distilled water (H2O). The 12% nitric acid solution was prepared by mixing a 60% nitric acid solution and distilled water (H2O) in a ratio of 1:4. The implant, from which surface impurities were removed by washing in the nitric acid solution, was dipped in distilled water, followed by ultrasonic treatment for 15 minutes.
Preparation of a calcium phosphate solution was initiated from a step of preparing high-concentration 200mM calcium (Ca) ion solution and phosphate (PO4) ion solution. The high-concentration calcium ion solution was prepared by dissolving calcium chloride (CaCl2) in distilled water (H2O), and the high-concentration phosphate ion solution was prepared by dissolving sodium hydrogen phosphate (Na2HPO4) in distilled water (H2O). The high-concentration 200mM calcium ion solution and phosphate ion solution were respectively diluted to 5mM by the addition of distilled water. 5mM calcium ion solution and 5mM phosphate ion solution were stored at the same temperature at which a coating process was to be carried out. Preferably, the calcium ion solution and the phosphate ion solution were stored in an incubator where the coating process was to be carried out.
The low crystalline hydroxyapatite coating was prepared in a manner that an implant finally pretreated in distilled water was placed in a reaction container, to which 5mM low-concentration phosphate ion solution and 5mM calcium ion solution were each sequentially added in an equal volume and mixed to prepare a 2.5mM calcium phosphate solution. The reaction container containing the titanium implant and the 2.5mM calcium phosphate solution was stored in an incubator at 20 °C for 60 minutes. Thereafter, the titanium implant was removed from the reaction container, washed briefly with distilled water, and dried to form a network-like coating surface.
A scanning electron micrograph of
In order to adjust a thickness of the coating, the implant can be stored in an incubator for 180 minutes or more to form a thick coating of low crystalline hydroxyapatite. When the calcium phosphate solution is first prepared and 1 to 3 minutes later, the pretreated implant is placed and stored in a reaction container for 60 minutes, it may also obtain an island-like coating surface externally existing both of titanium and low crystalline hydroxyapatite.
For an experimental group which was ultrasonic-pretreated in distilled water as in Example 1 and a non-pretreated control group, titanium implants were treated as in the procedure of Example 2 and Example 3.
In order to measure a degree of differentiation of mesenchymal stem cells, which were cultured in a cell culture dish, into osteoblasts, activity of alkaline phosphatase (ALP) of the cells was assayed for the implants coated with low crystalline hydroxyapatite prepared in the above Example.
Human bone marrow-derived mesenchymal stem cells were seeded into a RBM (surface treatment method using resorbable blasting media)-treated titanium disc which was coated with low crystalline hydroxyapatite. A RBM-treated titanium disc was used as a control group. In order to induce differentiation of mesenchymal stem cells into osteoblasts, the cells were treated with a culture medium supplemented with 0.1 µM dexamethasone, 10 mM β-glycerol phosphate and 50 µg/ml ascorbic acid, followed by culture for 14 days, and alkaline phosphatase activities were assayed and compared each other. The cultured cells were washed with PBS, lysed with a lysis buffer, and assayed using an ALP activity assay kit.
As shown in a graph of
In order to measure a degree of calcified nodule formation of mesenchymal stem cells which were cultured on a cell culture dish, the following experiment was carried out for the implants coated with low crystalline hydroxyapatite prepared in Example.
Human bone marrow-derived mesenchymal stem cells were seeded into a RBM (surface treatment method using resorbable blasting media)-treated titanium disc which was coated with low crystalline hydroxyapatite analogously to the procedure in Experimental Example 1. A RBM-treated titanium disc was used as a control group. In order to induce differentiation of the mesenchymal stem cells into osteoblasts, the cells were treated with a differentiation-inducing culture medium, followed by culture for 2 or 4 weeks. The cultured cells were washed with PBS, fixed with 4% paraformaldehyde for 15 minutes, and then washed with distilled water. An Alizarin Red solution (pH 4.2), which had been previously prepared, was added to the cells to perform a staining process for 20 minutes. The cells were then washed with distilled water to remove the non-stained solution. For quantitative evaluation of the stained calcified nodules, 10% w/v of cetylpyridinium chloride was added to a sodium phosphate solution (pH 7) to elute the stained dye which was then subjected to measurement of absorbance.
As shown in a graph of
The implant coated with low crystalline hydroxyapatite prepared in Example was transplanted into rabbit ilia and the survival of a coating film before and after bone remodeling was investigated.
A titanium implant, which was coated with low crystalline hydroxyapatite analogously to the procedure in Experimental Example 1, using an RBM (surface treatment method using resorbable blasting media)-treated titanium implant and a sample thereof as a base material, was transplanted into the ilium of a New Zealand white rabbit. The implant has a diameter of 3.5 mm and a length of 8.5 mm. For the grafting of the implant, the rabbit ilium was drilled to have a diameter of 3.6 mm and the implant was manually pushed into the ilium. After 2 and 6 weeks, the rabbit was sacrificed, and the implant was removed from the rabbit and examined under a scanning electron microscope.
The scanning electron micrograph given for comparative purposes in
As can be seen from the Experimental Examples as described above, the implant coated with low crystalline hydroxyapatite having a mixed type of network-like morphology and island-like morphology promotes cellular differentiation and calcified nodule formation. Taken together, it can be confirmed that the low crystalline hydroxyapatite-coated implant described herein exhibits excellent biocompatibility. Further, it can be seen that the implant described herein can be used as a superior biomaterial that has bioabsorbability by participation thereof in a bone remodeling process, and ameliorates peeling of the coating film and consequent separation of the implant.
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